Nitrous oxide is used as an anaesthetic gas in hospitals. However, if a doctor or nurse working in an operating room continuously inhales leaked nitrous oxide gas over a prolonged period, there is a fear that the doctor or nurse may have a health disorder. In addition, discharging the nitrous oxide gas ascends to the stratosphere, where it changes into nitrogen monoxide. The nitrogen monoxide reacts with ozone to form a substance which causes ozonosphere depletion like chlorofluorocarbon gases and the like. Furthermore, since nitrous oxide has a coefficient of global warming several hundred times that of carbon dioxide and the life thereof in the atmosphere is as long as about 150 years, it is a substance which exerts considerable influence on global warming. Inhibiting this harmful substance, nitrous oxide, from diffusing into the air is coming to be a future important need in hospitals, etc.
Although the leakage of nitrous oxide into an operating room is attributable to leakage from joints in the anaesthetic apparatus and to the excess discharge of the anaesthetic gas from the apparatus, it is mostly attributable to the latter. The currently employed measures for reducing the concentration of an anaesthetic gas in an operating room include (1) ventilating the room, (2) adsorptively removing the excess anaesthetic gas with activated carbon, (3) discharging the excess anaesthetic gas into the atmosphere with a suction type discharge apparatus, and (4) decomposing the excess anaesthetic gas with a decomposing apparatus after the gas is collected with a suction type discharge apparatus. Among these, method (2) has a drawback in that the amount of nitrous oxide gas adsorbable onto activated carbon is small, while method (3) has a drawback in that discharging the gas outside is undesirable from the standpoint of avoiding global warming. Consequently, method (4) is considered to be the best.
With respect to an apparatus for decomposing nitrous oxide gas, a technique is described in JP-B-1-45487 (the term "JP-B" as used herein means an "examined Japanese patent publication"), in which a catalyst comprising a silica powder having, deposited thereon, both nickel containing a platinum group metal and lanthanum oxide, is packed into a reactor and the gas to be decomposed is passed through the reactor heated at 150 to 550.degree. C. to decompose the same. However, this technique is undesirable in that this treatment is known to yield nitrogen dioxide and nitrogen monoxide, as by-products, which are harmful to the human body and cause environmental pollution. Another drawback thereof is that since the catalyst deteriorates at a high rate under such high-temperature conditions, the catalyst needs to be replaced with a fresh one in a short time period and periodic maintenance is necessary. Furthermore, it is extremely difficult to regenerate the deteriorated catalyst, so that the expense of discarding the catalyst cannot be avoided.